Buy zippers at fabric stores. Measure the length of the current zipper in the dress to determine the length of zipper you need to buy. Decide on the color of zipper you want. Usually a zipper that matches your dress fabric is the best choice -- when you bend or move and strain the seam that contains the zipper, the teeth and the fabric sides of the zipper may be exposed.
Removing Your Old Zipper
Using a seam ripper, carefully break the stitches holding the existing zipper in place. Take your time doing this so that you don't make a hole in your dress fabric. Sitting in a well-lit room will also help you see these small stitches. Once the zipper is removed, continue ripping the entire seam open.
Pinning the New Zipper in Place
With right sides of zipper and fabric together, place one unzipped side of the zipper's coils along the seam line and place the edge of the zipper tape along the fabric's edge. Pin this side of the zipper, and baste it into place by hand. Pin the other side in mirror arrangement, again having teeth along the seam line and the edge of the zipper tape along the fabric's edge. Baste it into place by hand. Remove all pins. Use a removable marking pencil to place a mark on the zipper tape by any seams you passed while sewing the zipper; this helps line up the seams once the stitching is complete.
Sewing the Zipper
Fit your sewing machine with its zipper foot. Place one side of the zipper tape under the needle in the sewing machine so that the zipper foot glides as close to the teeth as possible without sewing over them. Sew the full length of the zipper, stopping 5/8 inch below its top. Repeat on the other side. At the bottom, sew as close as you can to the zipper tab, stopping when the zipper foot hits it. Zip the zipper closed, flip the garment inside out, and check to ensure the zipper is aligned correctly. If the garment does not align, use the seam ripper to remove one side of the zipper and resew. Sew any gaps in the seam near the tab by hand.